Devices that convert light or radiation to charge signals and output the converted charge signals are used in various industrial fields. Such devices include photoelectric converters, such as CCDs, photomultipliers, and the like, radiation image recording devices that generate and store charges therein by receiving radiation and output charge signals according to the charges stored therein, and the like.
As for detectors for detecting the charge signals outputted from the photoelectric converters and radiation image recording devices described above, integrating amplifiers are generally used, since such amplifiers may be formed of ICs and have comparatively low noise. The integrating amplifier initiates integration of the charge signals and outputs an electrical signal according to the amount of integrated charges when switched to integration mode, and returns to initial state by discharging the integrated charge signals when switched to reset mode.
Here, the switching to the integration mode of the integrating amplifier is implemented by switching the reset switch on the integrating amplifier from ON to OFF. The switching of the reset switch causes kTC noise of the reset switch to be added to the signal component of the electrical signal. In order to avoid the influence of kTC noise, correlated double sampling is performed, as described, for example, in the non-patent document by R. L. Weisfield and N. Robert Bennett, “Electronic Noise Analysis of a 127-Micron Pixel TFT/Photodiode Array”, Proc. SPIE, vol. 4320, pp. 209-218, 2001. The correlated double sampling is a method for avoiding the influence of kTC noise by obtaining the difference between an electrical signal outputted from the integrating amplifier when a predetermined baseline sampling time is elapsed after it is switched to integration mode, and an electrical signal outputted from the integrating amplifier just before it is switched to reset mode, and using the difference as the signal component.
In a signal detection circuit that employs an integrating amplifier like that described above, a low-pass filter is provided at the latter stage of the integrating amplifier in order to reduce thermal noise generated in a signal line connected to the input terminal of the integrating amplifier, and the electrical signal outputted from the integrating amplifier is outputted through the filter.
Here, if the resistance value of the signal line connected to the integrating amplifier is, for example, several hundreds of kΩ, the thermal noise becomes significant with respect to a charge signal flowing through the signal line, since the amount of thermal noise increases with an increase in the resistance value of the signal line. In order to satisfactorily reduce the aforementioned noise, it is necessary to increase the time constant of the low-pass filter according to the amount of the noise.
Baseline sampling (acquisition of electrical signal at the end of the baseline sampling time) in such a correlated double sampling as described above is for sampling kTC noise of an integrating amplifier as described above, and not for reducing noise generated in the signal line described above. Therefore, it is not necessary to increase the time constant when the baseline sampling is performed.
Further, in order to obtain kTC noise with a sufficiently large magnitude in the baseline sampling, it is necessary to sufficiently increase the baseline sampling time with respect to the time constant of the low-pass filter, since the low-pass filter has a transient response characteristic. Thus, if the time constant of the low-pass filter is increased when the baseline sampling is performed as described above, the baseline sampling time will also need to be increased correspondingly, resulting in an unnecessarily prolonged time for signal detection.
The prolonged signal detection time described above makes it impossible to perform high-speed signal detection, for example, detection of semi-motion picture signals.
In view of the circumstances described above, it is an object of the present invention to provide a signal detection method and apparatus, and a radiation image signal detection method and system, which employ correlated double sampling and capable of appropriately reducing noise caused by the line resistance as described above, as well as increasing the speed of signal detection.